In homogeneous charge compression ignition (HCCI) engine, combustion is flameless, and spontaneously occurs at the entire cylinder volume. The homogeneously mixed cylinder charge is auto-ignited as the cylinder charge is compressed and its temperature increases. The ignition timing of auto-ignited combustion strongly depends on initial cylinder charge conditions such as temperature, pressure, and composition. Thus, it is important to coordinate the engine inputs, such as fuel mass, injection timing, and valve motion, to ensure robust HCCI combustion. Depending on the valve motion, there are two prevailing operating strategies in an HCCI engine—exhaust recompression strategy and exhaust re-breathing strategy.
In the exhaust recompression strategy, the cylinder charge temperature is controlled by trapping hot exhaust gas from the previous engine cycle by closing the exhaust valve early during the exhaust stroke, while opening the intake valve with late timing symmetrical to the exhaust valve closing timing. In this valve strategy, the cylinder charge composition and temperature depend on how early the exhaust valve closes during the exhaust stroke. For example, if the exhaust valve closes earlier during the exhaust stroke, more hot exhaust gas from the previous engine cycle would be trapped in the cylinder, leaving less cylinder volume for the fresh air mass, thereby, increasing the cylinder temperature, while decreasing the cylinder oxygen level. In the exhaust recompression strategy, the exhaust valve closing timing (thereby, the intake valve opening timing) is typically quantified by valve overlap, which has a negative number. The Negative Valve Overlap (NVO) is defined as the duration in crank angle between exhaust valve closing and intake valve opening. Thus, the initial cylinder charge conditions strongly depend on the intake and exhaust valve timings.
In contrast, in the exhaust re-breathing strategy, the cylinder charge temperature is controlled by re-inducting the hot exhaust gas into the cylinder through re-opening of the exhaust valve during the intake stroke. In this valve strategy, the cylinder charge composition and temperature depend on the lift of the exhaust valve re-opening during the intake stroke. Similarly with the exhaust recompression strategy, if the exhaust valve re-opens higher during the intake stroke, more hot exhaust gas from the previous engine cycle would be re-inducted in the cylinder, leaving less cylinder volume for the fresh air mass which, as a result, would increase the cylinder temperature, while decreasing the cylinder oxygen level. In this case, initial cylinder charge conditions strongly depend on the lift and/or duration of second opening of the exhaust valve.
Either with exhaust recompression strategy or re-breathing strategy, rapid change in inputs to the HCCI engine such as valve timing, EGR valve opening, injection timing, etc., is required to maintain sufficient thermal energy for successful auto-ignited combustions during load/speed transient. Thus, precise and fast actuator control is necessary for a successful transition between set-points, while those set-points are also required to be robust to disturbances that can be inevitably introduced during transient operation. Additionally, HCCI is sensitive to operating factors such as, for example, ambient temperature, engine coolant temperature, altitude, humidity, etc. making robust and stable control even more challenging.